Method for production of N-phenyl and N-cyclohexyl maleimides

ABSTRACT

A method for the production of maleimides, which comprises subjecting maleinamic acids to ring-closure imidation in an organic solvent capable of forming an azeotrope with water in the presence of an acid catalyst at a temperature in the range of 120° C. to 250° C. while removing the formed water in the form of an azeotrope with said organic solvent and thereafter purifying the produced maleimides.

This application is a division, of application Ser. No. 745,414, filedJune 14, 1985, U.S. Pat. No. 4,623,734.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for the production of maleimides.More particularly, this invention relates to a method for the productionof maleimides by ring-closure imidation of maleinamic acids.

2. Description of the Prior Art

Maleimides are compounds useful as raw materials for synthetic resins,medicines, and agricultural chemicals. Researches after methods fortheir production have long been under way. The most popular method ofthem all effects the production of maleimides by the dehydrationcyclization of maleinamic acids with a dehydrating agent such as aceticanhydride. One version of this method is disclosed in U.S. Pat. No.2,444,536. This method effects the production of maleimides by causingmaleic anhydride to react upon amines thereby forming maleinamic acidsand dehydration cyclizing and, at the same time, imidating themaleinamic acids in the presence of acetic anhydride and sodium acetate.This method, however, has the disadvantage that the imidation requiresexpensive acetic anhydride to be used in at least an equivalent relativeto the maleinamic acid and the separation and recovery of the formedmaleimide from the imidation reaction solution necessitates use of alarge volume of water and, as the result, entails disposal of a largeamount of an acetic acid-containing effluent at great expense. Thus,this method may well be called a too expensive method for commercialproduction of maleimides.

A method which has no use for such a chemical dehydration agent asacetic anhydride is disclosed in British Pat. No. 1,041,027. This methodeffects the production of maleimides by thermally dehydrating andcyclizing maleinamic acids in conjunction with a solvent such as, forexample, toluene, xylene, or chlorobenzene having a boiling pointexceeding 80° C. and serving as a diluent and an acid catalyst such assulfur trioxide, sulfuric acid, or ortho-phosphoric acid, and distillingthe system thereby causing azeotropic expulsion of the consequentlyformed water in conjunction with the solvent. As compared with themethod which uses acetic anhydride, this method proves advantageous inthat it does not require use of a large amount of such an expensivedehydrating agent as acetic anhydride and further that the formedmaleimides are separated and recovered with ease. This methodnevertheless has the disadvantage that the yield of the imidation is lowas compared with that obtainable by the method using acetic anhydride.This disadvantage is logically explained by a postulate that comparedwith the method which effects the imidation by the use of aceticanhydride, the method which effects the imidation by performing thermaldehydration in the specific solvent as described above involves a highreaction temperature and, therefore, tends to induce secondary reactionsand inevitably manages to produce maleimides abounding with impuritiesand further that since maleimides are thermally unstable, the maleimidesproduced at all are degenerated during the course of the reaction.

There is another method which, as disclosed in Japanese Patent Laid-openSHO No. 53(1978)-68,700 and Japanese Patent Publication SHO No.57(1982)-42,043, comprises causing maleic anhydride to react on aminesin the presence of an organic solvent thereby forming maleinamic acidsand subjecting the maleinamic acids as held in a state not isolated fromthe reaction system to dehydration and cyclization in the presence ofsuch an aprotic polar solvent as dimethyl formamide or dimethylsulfoxide and an acid catalyst. By this method, there is offeredrecognizable improvement in yield as compared with the second methoddescribed above. This method, however, has these disadvantages, that thecost of production of maleimides is high because expensive and highlytoxic aprotic polar solvent such as dimethyl formamide is used in alarge amount, that the solvent such as dimethyl formamide is degeneratedby the action of an acid catalyst used in the reaction and, therefore,the solvent is lost greatly, and that since the aprotic polar solventused in the reaction has a high boiling point, the solvent is removedfrom the produced maleimides with great difficulty.

Yet another method has been proposed (as in Japanese Patent Laid-openSHO No. 47(1972)-27,974) which effects the imidation of maleinamic acidsby directly heating the acids in the presence of an acid catalyst.Unfortunately, this method is not free from the following disadvantagesthat a large amount of resinous impurities is produced in the reactionvessel resulting to obtain maleimides in low yield, and that, moreover,the crude maleimides inevitably contain by-produced maleic acid in greatdeal.

In addition to the disadvantages described above, these methods for theproduction of maleimides are invariably attended by the essentialrequirement that since by-products occur in not negligible amountsduring the course of imidation, the produced maleimides should bepurified in order to acquire high purity at great expenses.

As means of purification such impure maleimides, there have beenproposed a method which comprises pouring the reaction solution into alarge amount of cold water thereby inducing precipitation of crystals,separating the crystals by filtration, washing the crystals with a largeamount of water or washing them with a dilute aqueous solution of sodiumcarbonate or aqueous solution of sodium hydroxide, and thereafter dryingthe washed crystals (U.S. Pat. No. 2,444,536 and Japanese PatentLaid-open SHO No. 55(1980)-149,253) and a method which comprisesneutralizing the reaction solution containing a maleimide in an organicsolvent with a dilute aqueous solution of a weak alkali and washing theneutralized reaction solution with water thereby separating the organicsolvent therefrom (Japanese Patent Laid-open SHO No. 53(1978)-68,770).

In accordance with these methods, however, it is difficult to obtainmaleimides of high purity because the by-products which occur during thecourse of imidation are as insoluble in water as resinous substances andmaleimides. Since these methods involve use of large amounts of water,they inevitably entail the problem of safe disposal of large amounts ofwashings. They are deficient in practical utility and provedisadvantageous from the economic point of view.

An object of this invention, therefore, is to provide a novel method forthe production of maleimides.

Another object of this invention is to provide a method forinexpensively producing maleimides in high yields by the ring-closureimidation of corresponding maleinamic acids effected by a safe andsimple procedure.

Still another object of this invention is to provide a method for theproduction of maleimides of high purity.

SUMMARY OF THE INVENTION

The objects described above are attained by the present inventionproviding a method for the production of maleimides, which comprisessubjecting maleinamic acids to ring-closure imidation in an organicsolvent capable of forming an azeotrope with water in the presence of anacid catalyst at a temperature in the range of 120° to 250° C. whileremoving the formed water in the form of an azeotrope with the organicsolvent and thereafter purifying the produced maleimides.

The reaction mentioned above is additionally carried out by effectingthe ring-closure imidation in the presence of a compound of at least onemetal selected from the group consisting of zinc, chromium, cobalt,nickel, iron, aluminum, and palladium and a stabilizer.

The purification of the produced maleimides are carried out by adding tothe crude maleimides organic acid or inorganic acid in an amount of atleast 1% by weight based on the maleinamic acids used as the rawmaterial, treating the resultant mixture at a temperature in the rangeof 5° to 100° C., and washing the resultant organic layer with waterthereby depriving the organic layer of water-soluble impurities.

Alternatively, the purification of the crude maleimides are effected bywashing the crude maleimides with water and distilling the washedmaleimides in the presence of a stabilizer.

DESCRIPTION OF PREFERRED EMBODIMENTS

The method for the production of maleimides according to this inventioncomprises subjecting maleinamic acids to ring-closure imidation in anorganic solvent capable of forming an azeotrope with water in thepresence of an acid catalyst at a temperature in the range of 120° to250° C. while removing the formed water in the form of an azeotrope withthe organic solvent and thereafter purifying the produced maleimides.

The maleinamic acids to be used in this invention are easily obtainedgenerally by the reaction of primary amines with maleic anhydride. Theyare desired to be compounds represented by the following general formulaI. ##STR1## wherein R denotes a member selected from the classconsisting of alkyl of 1 to 20 carbon atoms, phenyl, benzyl, cyclohexyl,pyridyl, and quinolyl groups, and the same groups as mentioned above andpossessed of halogen, carboxyl, or nitro substituents; providing thatsaid alkyl groups or phenyl groups are more desirable than the othergroups mentioned.

Examples of the primary amine particularly useful as the raw materialfor the maleinamic acid in this invention include methylamine,ethylamine, n-propylamine, isopropylamine, n-butylamine, sec-butylamine,isobutylamine, tert-butylamine, n-hexylamine, n-dodecylamine,allylamine, benzylamine, cyclohexylamine, aniline, nitroaniline,aminophenol, aminobenzoic acid, anisidine, ethoxyphenylamine,monochloroaniline, dichloroaniline, toluidines, xylidines, andethylanilines.

Synthesis of a maleinamic acid proceeds virtually stoichiometrically.For example, the maleinamic acid can be synthesized by causing the aminein an amount of 0.8 to 1.5 mols, preferably 0.9 to 1.2 mols, to reactupon each mol of maleic anhydride. If the amount of the amine relativeto that of the maleic anhydride exceeds the upper limit of theaforementioned range, the amount of an acid catalyst to be required whenthe product of this reaction is directly used in the subsequentimidation must be increased in a large excess and the amounts of theproducts of secondary reactions attendant on the imidation are increasedpossibly to the extent of lowering the yield of the imidation. Thus, therange mentioned above must be observed to ensure satisfactory results ofthe imidation.

The organic solvent capable of forming an azeotrope with water andusable for the purpose of this invention is desired to be a solventwhich permits the water formed in the dehydration cyclization to beremoved by azeotropic distillation and refrains from participating inthe reaction. Concrete examples which meet this description includebenzene, toluene, xylenes, ethylbenzene, isopropylbenzene, cumene,mesitylene, tert-butylbenzene, pseudocumene, trimethylhexane, octane,tetrachloroethane, nonane, chlorobenzene, ethyl cyclohexane, petroleumfractions boiling at 120° to 170° C., m-dichlorobenzene,secbutylbenzene, p-dichlorobenzene, decane, p-cymene, o-dichlorobenzene,butylbenzene, decahydronaphthalene, tetrahydronaphthalene, dodecane,naphthalene, cyclohexylbenzene, and petroleum fractions boiling at 170°to 250 ° C. For the purpose of enabling the reaction to proceed smoothlyand satisfying the economic conditions of the production, the amount ofthis solvent to be used in the reaction is desired to fall in the rangeof 1 to 20 times, preferably 3 to 7 times (by weight) the amount of themaleinamic acids.

Examples of the acid catalyst effectively used herein include sulfuricacid, sulfuric anhydride, p-toluenesulfonic acid, ortho-phosphoric acid,meta-phosphoric acid, and pyrophosphoric acid. The amount of the acidcatalyst is desired to fall in the range of 2 to 80 mol %, preferably 10to 30 mol %, based on the maleinamic acids.

In the reaction described above, a metal-containing compound is desiredto be used additionally as a promoter. The metal-containing compound isa compound of at least one metal selected from the group consisting ofzinc, chromium, palladium, cobalt, nickel, iron, and aluminum. Compoundsof zinc are desirable selection. Thus, the metal-containing compounds isspecifically selected from among oxides, acetates, maleates, succinates,nitrates, phosphates, chlorides, bromides, iodides, and sulfates of theaforementioned metals. Preferable compounds are acetic acid salts ofthese metals and zinc acetate is the most preferable. The amount of themetal-containing compound to be used is desired to fall in the range of0.005 to 0.5 mol, preferably 0.01 to 0.1 mol as metal per 100 mols ofthe maleinamic acids.

The aforementioned reaction produces even better results when it isallowed to proceed in the presence of a stabilizer. Examples of thestabilizer effectively usable herein include quinones such asmethoxybenzoquinone, hydroquinone, and tert-butyl hydroquinone; phenolssuch as p-methoxyphenol, tert-butyl catechol, alkyl phenols, and alkylbisphenols; thiodipropionic esters such as dilauryl thiodipropionate;dithiocarbamates such as zinc dimethyl dithiocarbamate and copperdibutyl dithiocarbamate; salycylates; alkylated diphenylamines;phenothiazines such as phenothiazine and methylene blue;mercaptoimidazoles such as 2-mercaptobenzimidazole; and triphenylphosphates. This stabilizer plays the part of enabling the maleimidesformed by the imidation to remain intact even at the elevatedtemperatures prevailing during the course of the imidation. If theamount of the stabilizer to be used is very small, the added stabilizeris not so effective as expected. If the amount is excessive, thereensues the problem that the added stabilizer mingles into the product ofthe reaction. Thus, the amount of the stabilizer so used is desired tofall in the range of 0.005 to 0.5 mol, preferably 0.05 to 0.3 mol, per100 mols of the maleinamic acids.

The first step in the practice of the method of this invention is to addan amine to a solution of maleic anhydride in an organic solvent andheat the resultant mixture at a temperature not exceeding 150° C.,preferably falling in the range of 30° to 120 ° C. for a period of 15 to120 minutes, preferably 30 to 60 minutes, so as to effect reaction ofthe maleic anhydride with the amine and give rise to a correspondingmaleinamic acids. The next step is to heat the maleinamic acids in astate not isolated from the resultant reaction solution, with theaforementioned acid catalyst and optionally the metal-containingcompound and the stabilizer added thereto, at a temperature in the rangeof 120° to 250° C., desirably 140° to 220 ° C. for a period of 1 to 15hours, desirably 2 to 7 hours and continue the ensuing reaction whileremoving the formed water from the system by azeotropic distillation. Asthe result, corresponding maleimides are obtained in a high yield.Particularly when the reaction temperature is kept in the range of 180°to 250° C., since accelerating the reaction velocity the selectivity ofthe maleimides is notably improved without requiring the combined use ofthe metal-containing compound and the stabilizer. The effect of the useof the temperature in the specific range is conspicuous when themaleimides to be produced is of a type possessing an aromatic typesubstituent, and alicyclic type substituent, or an aliphatic typesubstituent. The solvent to be used in the reaction is selected in dueconsideration of the ability to dissolve the produced maleimides, theprice, the ease of handling, and the possession of a boiling pointmeeting the reaction conditions. Under certain reaction conditions, thesolvent may be advantageously used under increased pressure. Thestabilizer can also be selected in due consideration of the reactionconditions and the kind of the maleimides to be produced.

The maleimide which is consequently obtained is a compound representedby the general formula II, for example. ##STR2## wherein R has the samemeaning as defined above. Typical examples of the maleimides includeN-methyl maleimide, N-ethyl maleimide, N-n-propyl maleimide, N-isopropylmaleimide, N-n-butyl maleimide, N-sec-butyl maleimide, N-tert-butylmaleimide, N-n-hexyl maleimide, N-n-dodecyl maleimide, N-allylmaleimide, N-benzyl maleimide, N-cyclohexyl maleimide, N-phenylmaleimide, N-nitrophenyl maleimide, N-hydroxyphenyl maleimide,N-methoxyphenyl maleimide, N-ethoxyphenyl maleimide, N-monochlorophenylmaleimide, N-dichlorophenyl maleimide, N-monomethylphenyl maleimide,N-dimethylphenyl maleimide, and N-ethylphenyl maleimide. Of course, themaleimides which this invention is intended to embrace are not limitedto the examples cited above.

The crude maleimides which are produced in consequence of the imidationdescribed above is purified by the following procedure, for example. Thepurification of the crude maleimides are accomplished by adding to thecrude maleimides an organic acid or inorganic acid in an amount of atleast 1% by weight based on the maleinamic acids used as the rawmaterial, treating the resultant mixture at a temperature in the rangeof 5° to 100° C. and then washing the resultant organic layer with waterthereby depriving it of water-soluble impurities.

For the acid used in this acid treatment to be fully effective asexpected, the distinction between the organicity and inorganicity of theacid is irrelevant. Typical examples of the acid advantageously usableherein include ortho-phosphoric acid, pyrophosphoric acid, sulfuricacid, sulfuric anhydride, fuming sulfuric acid, methane sulfonic acid,chlorosulfonic acid, chloromaleic acid, paratoluenesulfonic acid, andtrifluoroacetic acid. Among other acids, sulfonic acids, sulfuric acid,sulfuric anhydride, and fuming sulfuric acid are desirable selections.One member or a mixture of two or more members selected from amont theacids enumerated above can be used effectively in this invention.

The amount of the acid to be used herein is a matter to be suitablydecided in due consideration of the amount of impurities in the crudemaleimides, the purity of the product to be obtained, the yield of thepurification, the efficiency of work, and the economy of the production.This amount is not critical to the successful practice of the method ofthis invention. When any ordinary crude maleimides are purified inaccordance with this invention, the effect of the acid treatment isobtained so long as the amount of the acid so used is at least 1% byweight based on the maleinamic acids used as the raw material. The upperlimit of this amount can be fixed substantially at a freely selectedvalue. Where the yield of purification, the efficiency of work, theeconomy of production, and so on are taken into consideration, thisamount may be fixed at any level not exceeding 30% by weight. Preferablythis amount falls in the range of 5 to 10% by weight.

The acid treatment is carried out at a temperature in the range of 5° to100° C., preferably 20° to 70° C., and in the time of 5 to 120 minutes,preferably 10 to 60 minutes. If this treatment is carried out at atemperature exceeding 100° C., the yield is substantially decreasedbecause secondary treatments occur so heavily to require a specialtreatment for their removal and the produced maleimides undergoespolymerization. If this treatment is carried out at a temperature lowerthan 5° C., the purity of the produced maleimides and the yield ofproduction are not sufficient because the effect of the treatment is notample and the maleimides produced in the reaction solution isprecipitated and removed in conjunction with by-products.

Even when sulfuric acid is used as an acid catalyst in the imidation,the acid treatment requires new addition of sulfuric acid in the amountdefined above. The acid catalyst to be used in the imidation is in anamount falling within the range of 10 to 30 mol % based on themaleinamic acids used as the raw material. If, at this time, the amountof the sulfuric acid put to use in the imidation has been increased withthe amount of sulfuric acid estimated to be used in the subsequent stepof acid treatment, the purification proves disadvantageous from thecommercial point of view because the yield of the production of themaleimides are notably low. This acid treatment is not sufficientlyeffective when it is carried out after the reaction mixture resultingfrom the imidation has been washed with water or when the organic layerobtained by separating the acid catalyst layer from the reaction mixturehas been washed with water.

By carrying out the acid temperature specified by this invention asdescribed above, the by-products formed during the course of thering-closure imidation and contained consequently in the reactionmixture is enabled to be precipitated substantially in its entire amountin the form of a viscous resinous substance from the reaction mixture.This precipitate can be easily separated from the reaction mixture by anordinary means such as filtration or decantation. Consequently, there isobtained an organic layer containing the produced maleimidessubstantially exclusively. Even when the acid treatment is carried outdirectly on the reaction mixture in a state not deprived of the acidcatalyst used in the ring-closure imidation, the treatment is aseffective as when it is carried out after the reaction mixture has beendeprived of the acid catalyst.

When the organic layer obtained in consequences of the acid treatment iswashed in water of an amount 1 to 5 times (by weight) the amount of themaleinamic acids used as the raw material at a temperature in the rangeof 10° to 50° C., the unaltered acid and the by-products remaining inminute amounts in the organic layer are removed. The water to be used inthis washing treatment may be pure water, service water, or city water.Otherwise, weakly alkaline water or weakly acidic water may be used.This washing treatment is required to lower the acid content in theorganic layer below 0.5% by weight, preferably below 0.1% by weight.Thereafter, the maleimides containing substantially no impurity can besafely obtained by expelling the organic solvent from the organic layerby distillation.

By the washing treatment performed in the present step, the impuritiesstill remaining in minute amounts in spite of the acid treatment areremoved and the unaltered acid is removed at the same time. During thecourse of the separation by vaporization of the organic solvent from thereaction solution resulting from the acid treatment, if part of the acidis suffered to remain in the reaction solution, it induces the producedmaleimide to undergo polymerization and brings about a decrease in thepurity of the maleimides. It is suspected that the heat evolved by thepolymerization will cause troubles and possibly jeopardize the safety ofthe operation involved.

The crude maleimides can be purified otherwise by the followingprocedure, for example. The purification of the crude maleimides areattained by washing with water the crude maleimides resulting from thering-closure imidation of the maleinamic acids and then distilling thewashed crude maleimides in the presence of a stabilizer.

Concerning the work of purifying the crude maleimides by distillation,the inventors studied various distillation conditions with a view todeveloping a solution capable of overcoming the high susceptibility topolyermization which the maleimides inevitably manifests owing to itspeculiar molecular structure. They have consequently found that theunaltered or by-produced maleic acid, the unaltered maleinamic acids,the acid used as the catalyst, and the acid component such as aceticanhydride or acetic acid used as the dehydrating agent which arecontained as impurities in the crude maleimides, go to promote thepolymerization of the maleimides during the distillation. Particularlywhen the acid component is suffered to remain in the crude maleimides ina concentration exceeding 1% by weight, a level variable more or lesswith the kind of the N-substituent of the maleimides, this acidcomponent remarkably promotes the polymerization and, at times, even gothe length of causing the maleimide to be polymerized virtually whollyduring the distillation.

The inventors, based on the discovery described above, continued a studyin search of a method for removal of the mischievous acid component.They have consequently found that this removal of the acid component iseasily attained by washing the crude maleimides with water and that thecrude maleimides resulting from the washing treatment is refined to highpurity in a high yield during the distillation because it is effectivelyprevented from the otherwise possible polymerization due to the heat ofthe distillation. The washing treatment mentioned aobve sufficientlyfulfils its purpose by lowering the concentration of the acid componentin the crude maleimides generally below 0.5% by weight, a level variablewith the kind of the N-substituent of the maleimides. Preferably, theconcentration is lowered by this treatment below 0.1% by weight. Themanner of the washing can be freely selected in due consideration of thecondition of the mixture which has just resulted from the reaction.Where the crude maleimides are obtained in a solid state at the end ofthe reaction, for example, the removal of the acid component isefficiently effected by first reducing the crude maleimides to apowdered form and then washing the powdered crude maleimides with water.Where the reaction has involved use of an organic solvent, the removalof the acid component is accomplished by giving the washing treatment tothe reaction solution after completion of the reaction. The water to beused for this washing treatment is not limited to pure water, servicewater, and city water. It may be weakly alkaline or weakly acidic water.

The crude maleimides which have undergone the washing treatment are thenmixed with a stabilizer and distilled. Examples of the stabilizereffectively usable herein include phenols such as methoxybenzoquinone,hydroquinone, tert-butyl hydroquinone; phenols such as p-methoxyphenol,tert-butyl catechol, alkly phenols, and alkyl bisphenols;thiodipropionic esters such as dilauryl thiopropionate; dithiocarbamatessuch as zinc dimethyl dithiocarbamate and copper dibutyldithiocarbamate; salycylates; alkylated diphenylamines; phenothiazinessuch as phenothiazone and methylene blue; mercaptoimidazoles such as2-mercaptobenzimidazole; and triphenyl phosphates. From the groupindicated above, a suitable stabilizer is selected by taking into dueconsideration the kind of the maleimides under treatment and therequirement that the stabilizer should avoid mingling into the extractunder the distillation conditions adopted.

According to the results of the inventors' study, the stabilizerselected from among copper dimethyldithiocarbamate, zincdimethyldithiocarbamate, copper dibutyldithiocarbamate, and coppersalicylate, particularly copper dibutyldithiocarbamate, remains in thebottoms and defies expulsion during the course of the distillation andmanifests a conspicuous effect in preventing the maleimide fromundergoing polymerization.

The amount of the stabilizer to be added to the maleimides undertreatment are not specifically defined. The stabilizer is used in anamount generally exceeding 10 ppm (by weight) and desirably falling inthe range of 100 to 2000 ppm (by weight), preferably 500 to 1000 ppm (byweight).

The conditions of the distillation are freely selected on the basis ofthe relation between the temperature and vapor pressure of themaleimide. Since the susceptibility of the maleimides to polymerizationgrows with the heightening temperature, it is advantageous to carry outthe distillation at as low a temperature as permissible.

Now, the present invention, will be described more specifically belowwith reference to working examples. It is to be understood naturallythat the present invention is not limited to these working examples.

EXAMPLE 1

A glass flask having an inner volume of 1 liter was fitted with athermometer, a stirrer, and a water separator. Then, a solution of 53 gof powdered maleic anhydride in 50 g of tetrahydronaphthalene was placedin the glass flask. To the glass flask whose interior temperature wasadjusted in advance to 90° C., a solution of 50 g of aniline in 400 g oftetrahydronaphthalene was added gradually and piecemeal over a period of30 minutes to synthesize a tetrahydronaphthalene slurry of N-phenylmaleinamic acid. The yield of N-phenyl maleinamic acid based on anilinewas 99.4 mol %.

The slurry containing the N-phenyl maleinamic acid and 10 g ofortho-phosphoric acid added thereto were heated at 210° C. for threehours. The resultant reaction mixture was cooled to 30° C., washed withwater, and distilled under a vacuum to expel tetrahydronaphthalene andobtain 78 g of N-phenyl maleimide crystals. By liquid chromatography,the crystals were found to have purity of 87.3% by weight, indicatingthe yield thereof to be 73.2 mol %.

EXAMPLE 2

A glass flask having an inner volume of 3 liters was fitted with athermometer, a stirrer, and a water separator. In this reactor, 240 g ofa solution containing 120 g of maleic anhydride in xylene was placed.Then, a solution containing 220 g of dodecylamine in 1880 g of xylenewas added gradually and piecemeal over a period of 120 minutes at 40° C.to the reactor. After the addition was completed, the mixture in thereactor and 65.4 g of orthophosphoric acid (purity 89% by weight) addedthereto were heated at 215° C. for 1 hour under 4.4 kg/cm². G to effectreaction. The resultant reaction mixture was cooled to 30° C., washedwith 400 ml of water, and separated into a water layer and a xylenelayer. The xylene layer was separated by filtration to effect removal ofinsolubles. The xylene layer was distilled to expel xylene.Consequently, there was obtained 258 g of slightly impure N-dodecylmaleimide. This product was found to have purity of 85.3% by weight,indicating the yield to be 69.9 mol % based on dodecylamine.

EXAMPLE 3

A vertical reaction tank measuring 600 cm in inside diameter and 800 cmin height and fitted with a thermometer, a heating jacket, a stirrer anda water separator was charged with a mixed solution comprising 15 kg ofmaleic anhydride and 30 kg of xylene and heated to an internaltemperature of 70° C. Then, to the reaction tank, a mixed solutioncontaining 13.5 kg of aniline and 70 kg of xylene was added graduallyand piecemeal over a period of 100 minutes. Consequently, there wasobtained a slurry solution containing white crystals of N-phenylmaleimide.

Subsequently, 3.2 kg of ortho-phosphoric acid (purity 89% by weight),9.6 g of zinc acetate, and 26.7 g of p-methoxypehnol were added.

Then, the internal temperature of the reaction tank was elevated to 136°C. and kept at this level for six hours by means of the heating jacketto promote the ensuing reaction. The water formed in the meantime wasdistilled in conjunction with xylene. The total amount of water thusexpelled was 2.5 liters.

Then, the resultant reaction mixture was cooled to 30° C. and mixed with30 liters of water added thereto for one hour. After the stirring, thereaction mixture was left standing for 30 minutes to be separated into axylene layer and a water layer. Then the water layer was separated. Theprocedure was repeated once more. The xylene layer so washed with waterwas distilled under a vacuum to expel xylene. Consequently, there wereobtained 23 kg of N-phenyl maleimide crystals. By liquid chromatography,the crystals were found to have purity of 93.6% by weight.

Thus, the yield of the crystals is calculated to be 85.7 mol % based onaniline as the raw material.

EXAMPLE 4

The procedure of Example 3 was repeated, except that butylbenzene wasused in the place of xylene as the solvent, phenothiazone was used inthe place of p-methoxyphenol, and the reaction temperature was changedfrom 136° C. to 185° C. Consequently, there were obtained 24.7 kg ofN-phenyl maleimide crystals. By liquid chromatography, the crystals werefound to have purity of 90.8% by weight.

Thus, the yield of the crystals is calculated to be 89.3 mol % based onaniline used as the raw material.

EXAMPLES 5-21 AND CONTROL 1

A glass flask having an inner volume of 1 liter was fitted with athermometer, a stirrer, and a water separator. Then, a solution of 53 gof powdered maleic anhydride in 50 g of xylene was added to theaforementioned glass flask. To the glass flask whose internaltemperature was adjusted in advance to 130 ° C., a solution of 50 g ofaniline in 400 g of xylene was added gradually and piecemeal over aperiod of 30 minutes to synthesize a slurry xylene solution of N-phenylmaleinamic acid. The yield of the N-phenyl maleinamic acid was found tobe 99.2 mol % based on aniline.

The slurry solution so obtained and a varying acid catalyst, a varyingmetal-containing compound, and a varying stabilizer added therein invarying amounts based on the N-phenyl maleinamic acid contained in theslurry solution as indicated in Table 1 were heated at 140° C. for threehours to effect reaction. The reaction mixture consequently formed wascooled to 30° C., washed with water, and distilled under a vacuum toexpel xylene. Consequently, there were obtained N-phenyl maleimidecrystals. The crystals were tested by liquid chromatography for purity.The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                                  Yield (mol %)                                  Metal containing    Amount     (based on                       Acid catalyst  compound   Stabilizer                                                                             produced                                                                           Purity                                                                              maleinamic                      (mol %)        (mol % as metal)                                                                         (mol %)  (g)  (% by wt.)                                                                          acid)                           __________________________________________________________________________    Control 1                                                                           Ortho-phosphoric                                                                       Zinc Acetate 0                                                                           0        82   74.8  65.9                                  acid 20                                                                 Example 5                                                                           Ortho-phosphoric                                                                       Zinc Acetate 0.03                                                                        0        86   84.7  78.3                                  acid 20                                                                 Example 6                                                                           Ortho-phosphoric                                                                       Zinc Acetate 0.01                                                                        p-methoxy-                                                                             83   82.4  73.5                                  acid 20             phenol 0.1                                          Example 7                                                                           Ortho-phosphoric                                                                       Zinc Acetate 0.03                                                                        p-methoxy-                                                                             85   93.1  85.1                                  acid 20             phenol 0.1                                          Example 8                                                                           Ortho-phosphoric                                                                       Palladium  p-methoxy-                                                                             85   86.2  78.8                                  acid 20  nitrate 0.03                                                                             phenol 0.1                                          Example 9                                                                           Ortho-phosphoric                                                                       Aluminum oxide 0.03                                                                      p-methoxy-                                                                             83   78.7  70.2                                  acid 20             phenol 0.1                                          Example 10                                                                          Ortho-phosphoric                                                                       Chromium oxide                                                                           p-methoxy-                                                                             86   87.2  80.6                                  acid 20  (III) 0.03 phenol 0.1                                          Example 11                                                                          Ortho-phosphoric                                                                       Zinc oxide 0.03                                                                          p-methoxy-                                                                             84   91.2  82.4                                  acid 20             phenol 0.1                                          Example 12                                                                          Ortho-phosphoric                                                                       Cobalt     p-methoxy-                                                                             82   78.5  69.2                                  acid 20  phosphate 0.03                                                                           phenol 0.1                                          Example 13                                                                          Ortho-phosphoric                                                                       Nickel sulfate 0.03                                                                      p-methoxy-                                                                             83   79.0  70.5                                  acid 20             phenol 0.1                                          Example 14                                                                          Ortho-phosphoric                                                                       Iron oxide(II) 0.03                                                                      p-methoxy-                                                                             81   82.3  71.7                                  acid 20             phenol 0.1                                          Example 15                                                                          Sulfuric acid 20                                                                       Zinc acetate 0.03                                                                        p-methoxy-                                                                             68   92.1  67.3                                                      phenol 0.1                                          Example 16                                                                          P-toluenesulfonic                                                                      Zinc acetate 0.03                                                                        p-methoxy-                                                                             80   89.6  77.1                                  acid 20             phenol 0.1                                          Example 17                                                                          Ortho-phosphoric                                                                       Zinc acetate 0.03                                                                        Phenothiazine 0.1                                                                      84   92.2  83.3                                  acid 20                                                                 Example 18                                                                          Ortho-phosphoric                                                                       Zinc acetate 0.03                                                                        Hydroquinone 0.1                                                                       82   91.3  80.5                                  acid 20                                                                 Example 19                                                                          Ortho-phosphoric                                                                       Zinc acetate 0.005                                                                       p-methoxy-                                                                             83   81.4  72.6                                  acid 20             phenol 0.1                                          Example 20                                                                          Ortho-phosphoric                                                                       Zinc acetate 0.05                                                                        p-methoxy-                                                                             84   90.9  82.1                                  acid 20             phenol 0.1                                          Example 21                                                                          Ortho-phosphoric                                                                       Zinc acetate 0.03                                                                        p-methoxy-                                                                             83   90.1  80.4                                  acid 20             phenol 0.01                                         __________________________________________________________________________

EXAMPLES 22-25

The procedure of Example 7 was repeated, except that the kind of solventand the temperature were varied as indicated below. The results areshown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Ex-             Temper-  Amount                                               am-             ature    produced                                                                             Purity  Yield                                 ple  Solvent    (°C.)                                                                           (g)    (% by wt.)                                                                            (mol %)                               ______________________________________                                        22   P-cymene   180      90.5   91.5    89.0                                  23   Mesitylene 170      88.0   93.4    88.4                                  24   Tetrahydro-                                                                              210      83.8   94.4    85.1                                       naphthalene                                                              25   Dodecane   220      88.1   85.1    80.6                                  ______________________________________                                    

EXAMPLE 26

The procedure of Example 7 was followed, except that the temperature forthe imidation was increased to 148° C. Consequently, there were obtained91 g of yellow crystals.

Then, the crystals and 1700 ml of cyclohexane added thereto were heatedat 75° C. for one hour to extract N-phenyl maleimide. The extract wasfiltered. The filtrate was distilled to expel cyclohexane. Consequently,there were obtained 82 g of bright yellow crystals of N-phenylmaleimide. The purity of the crystals was found to be 99.4% by weight,indicating the yield thereof to be 87.6 mol % based on the N-phenylmaleinamic acid.

CONTROL 2

When the procedure of Example 7 was repeated, except that the additionof the stabilizer and the metal-containing compound was omitted, heavydecomposition was observed to occur during the course of the reaction.The amount of water formed in consequence of the reaction was 16 ml.This value represents an excess of 65 mol % over the theoretical valueof the amount of water to be formed by the imidation. The excess wateris believed to have been formed by some secondary reaction. Further inthe xylene solution, a brown fluffy substance appearing to be a polymerformed heavily. When this xylene solution was washed with water anddistilled to expel xylene, there was obtained 80 g of a brown mass ofN-phenyl maleimide. This mass was found to have purity of 70.3% byweight, inicating the yield thereof to be 60.5 mol % based on theN-phenyl maleinamic acid.

EXAMPLE 27

The same reactor as used in Example 7 was adopted. This reactor wascharged with a solution 53 g of maleic anhydride in 200 g ofchlorobenzene. Then, methylamine gas was blown in gradually andpiecemeal at 30° C. at a rate of 4.3×10⁻³ mol/min. over a period of twohours to produce a white slurry of N-methyl maleinamic acid.Subsequently, the slurry and 22.5 g of ortho-phosphoric acid (purity 89%by weight), 0.1 g of phenothiazine, and 0.02 g of zinc acetate addedthereto were heated at 134° C. for two hours to effect reaction.Thereafter, the resultant reaction mixture was cooled to 40° C.,filtered, and distilled under a vacuum to expel chlorobenzene.Consequently, there was obtained 45 g of light yellow N-methyl maleimidehaving purity of 92.1% by weight. The yield of the product is calculatedto be 72.5 mol % based on methylamine as the raw material.

By further distilling the impure N-methyl maleimide under vacuum of 35mmHg (abs) at 128° C., there were obtained 38.5 g of white N-methylmaleimide crystals. The crystals were found to have purity of 99.6% byweight, indicating the yield thereof to be 67.1 mol % based onmethylamine as the raw material.

EXAMPLE 28

A glass flask having an inner volume of 3 liters was fitted with atheremometer, a stirrer and a water separater. This reactor was chargedwith 240 g of a xylene solution containing 120 g of maleic anhydride.Then, a solution of 220 g of dodecylamine in 1880 g of xylene was addedthereto gradually and piecemeal at 40° C. over a period of 120 minutes.After the addition was completed, the resultant mixture and 65.4 g ofortho-phosphoric acid (purity 89% by weight), 0.672 g ofp-methoxyphenol, and 0.05 g of zinc acetate added thereto were heated at150° C. for four hours to effect reaction. Thereafter, the resultantreaction mixture was cooled to 30° C., washed with 400 ml of water, andseparated into a water layer and a xylene layer. The xylene layer wasfiltered to effect removal of insolubles. The xylene layer now free frominsolubles was distilled to expel xylene. Consequently there wasobtained 260 g of slightly impure N-dodecyl maleimide. It was found tohave purity of 91.2% by weight, indicating the yield thereof to be 75.3mol % based on dodecylamine.

This impure N-dodecyl maleimide was distiled under a vacuum of 5 mmHg(abs) at 180° C. Consequentley, there was obtained 207 g of whiteN-dodecyl maleimide crystals. The crystals were found to have purity of99.0% by weight, indicating the yield thereof to be 65.1 mol % based ondodecylamine as the raw material.

EXAMPLE 29

The same reactor as used in Example 7 was adopted. In this reactor, asolution of 120 g of maleic anhydride in 360 g of tetrahydronaphthalenewas placed. Then, a solution of 220 g of dodecylamine and 1760 g oftetrahydronaphthalene was added thereto at 40° C. over a period of 120minutes. After the addition was completed, the resultant mixture and65.4 g of ortho-phosphoric acid (purity 89% by weight), 1.344 g ofmethoxybenzoquinone, and 0.1 g of zinc acetate added thereto were heatedat 210° C. for one hour to effect reaction. Thereafter, the resultantreaction mixture was cooled to 30° C. and separated into a water layerand a tetrahydronaphthalene layer. The tetrahydronaphthalene layer wasdistilled to expel tetrahydronaphthalene. Consequently, there wasobtained 285 g of slightly impure N-dodecyl maleimide. It was found tohave purity of 92.3% by weight, indicating the yield thereof to be 83.5mol % based on dodecylamine.

EXAMPLE 30

A flask provided with a thermometer, a condenser incorporating amoisture separator, a dropping funnel, and a stirrer was charged with100 g of a petroleum fraction containing not less than 98% of anaromatic hydrocarbon having boiling points of 180° to 220° C. Thepetroleum fraction and 100 g of maleic anhydride added thereto wereheated by elevating the internal temperature of the flask to 70° C. todissolve the maleic anhydride in the petroleum fraction.

Then, a solvent of 90 g of aniline in 630 g of the same solvent asdescribed above was added dropwise while under stirring over a period of30 minutes to synthesize a slurry solution of N-phenyl maleinamic acidin the aforementioned solvent. During the course of this synthesis, theliquid temperature in the flask was kept cooled to 70° C. to repressotherwise possible evolution of heat.

Then, the aforementioned slurry solution and 20 g of ortho-phosphoricacid (85 weight% aqueous solution) added thereto were heated and stirredat 210° C. and left reacting for three hours, with the water formed bythe reaction removed from the reaction system by azeotropic distillationwith the aforementioned solvent. After the reaction was completed, thereaction solution was cooled to 100° C. From this reaction solution, theacid catalyst layer which settle to the bottom of the reactor wasseparated and removed.

The remaining reaction solution was further cooled to 30° C. and thecooled reaction solution and 15 g of sulfuric acid (purity not less than98% weight) added thereto were stirred for 30 minutes. This treatmentwith sulfuric acid educed a resinous substance from the reactionsolution. This resinous substance was separated from the reactionsolution. Thereafter, the remaining reaction solution and 185 g of wateradded thereto were stirred for 15 minutes and then the water layer wasseparated.

Finally, the organic layer was distilled under a vacuum of 5 mmHg toexpel the solvent. Consequently, there were obtained 118 g of yellowneedle crystals of N-phenyl maleimide.

By high performance liquid chromatography, the crystals were found tohave purity of 99.8% by weight, indicating the yield thereof to be 70.4mol % based on aniline.

CONTROL 3

The procedure of Example 30 was repeated, except that the treatment withsulfuric acid was omitted and the reaction solution freed from the acidcatalyst layer was directly washed with water and distilled to expel thesolvent. The N-phenyl maleimide consequently obtained was found to havepurity of 91.2% by weight.

EXAMPLE 31

A flask provided with a thermometer, a condenser incorporating a waterseparator, a dropping funnel, and a stirrer was charged with 100 g of apetroleum fraction containing not less than 98% of an aromatichydrocarbon having boiling points of 180° to 220° C. The petroleumfraction and 100 g of maleic anhydride added thereto were heated byelevating the internal temperature of the flask to 100° C. to dissolvethe maleic anhydride in the petroleum fraction.

Then a solution of 100 g of cyclohexylamine in 600 g of the same solventas described above was added dropwise while under stirring over a periodof one hour to synthesize a slurry solution of N-cyclohexyl maleinamicacid in the aforementioned solvent.

Then, the aforementioned slurry solution and 60 g of ortho-phosphoricacid added thereto were heated and stirred at 210° C. for two hours toeffect reaction, with the water formed by the reaction removed from thereaction system by azeotropic distillation with the solvent. After thereaction was completed, the reaction solution was heated at 200° C. tocause precipitation of an acid catalyst in the bottom. From the reactionsolution, the acid catalyst layer was separated and removed.

Then, the remaining reaction solution was cooled to 60° C. and thecooled reaction solution and 100 g of water added thereto were stirredfor 30 minutes. The water layer consequently formed was separated. Thisprocedure of washing was repeated once more. The organic layer wasdistilled under a vacuum of 3 mmHg (abs) to expel the solvent.

Then, the residue of distillation in the flask and 0.3 g of copperdibutyl dithiocarbamate added thereto were distilled under a vacuum of 5mmHg (abs) for 30 minutes, with the internal temperature of the flaskkept at temperatures of 130° to 150° C. to isolate N-cyclohexylmaleimide. Consequently, there were obtained 137 g of bright whiteN-cyclohexyl maleimide crystals. The crystals were found to have purityof 99.8% by weight, indicating the yield thereof to be 75.7 mol % basedon cyclohexylamine as the raw material.

EXAMPLE 32

A flask provided with a thermometer, a condenser incorporating a waterseparator, a dropping funnel, and a stirrer was charged with 100 g ofortho-xylene. The solvent and 100 g of maleic anhydride added theretowere heated by elevating the internal temperature of the flask to 70° C.to dissolve the maleic anhydride in the solvent.

Then, a solution of 90 g of aniline in 630 g of ortho-xylene was addedthereto dropwise while under stirring over a period of 30 minutes tosynthesize a slurry solution of N-phenyl maleinamic acid inortho-xylene. During the course of this synthesis, the liquidtemperature inside the flask was kept cooled to 70° C. to avoid theotherwise possible evolution of heat due to the dropwise addition of thesolution of aniline in ortho-xylene.

Then, the aforementioned slurry solution and 20 g of ortho-phosphoricacid (85 wt. % aqueous solution), 0.06 g of zinc acetate, and 0.2 g ofp-methoxyphenol added thereto were heated and stirred at 140° C. forthree hours, with the water formed by the reaction removed from thereaction system by azeotropic distillation with ortho-xylene. After thereaction was completed, the reaction solution was cooled to 100° C. andthe acid catalyst layer consequently separated from the reactionsolution to the bottom was separated and removed.

Subsequently, the remaining reaction solution was further cooled to 30°C. and the cooled reaction solution and 15 g of sulfuric acid (purity ofnot less than 98 wt. %) added thereto were stirred for 30 minutes. Thistreatment with sulfuric acid induced eduction of a resinous substancefrom the reaction solution. This resinous substance was separated fromthe reaction solution. The remaining reaction solution and 185 g ofwater added thereto were stirred for 15 minutes to effect separation ofa water layer.

Finally, the organic layer was distilled under a vacuum of 30 mmHg toexpel ortho-xylene. Consequently, there were obtained 135 g of yellowneedle crystals of N-phenyl maleimide.

By high performance liquid chromatography, the crystals were found tohave purity of 99.5% by weight, indicating the yield thereof to be 80.3mol % based on aniline.

CONTROL 4

The procedure of Example 32 was repeated, except that the treatment withsulfuric acid was omitted and the reaction solution freed from the acidcatalyst layer was directly washed with water and distilled to expelorthoxylene. The N-phenyl maleimide consequently obtained was found tohave purity of 90.0% by weight.

EXAMPLE 33

In a flask provided with a thermometer, a condenser incorporating awater separator, and a stirrer, 100 g of N-phenyl maleinamic acid and400 g of ortho-xylene were stirred. Then, the resultant solution and 10g of ortho-phosphoric acid (85 wt. % aqueous solution), 0.03 of zincacetate, and 0.1 g of p-methoxyphenol added thereto were heated andstirred at 140° C. for three hours, with the water formed by the ensuingreaction removed by azeotropic distillation with ortho-xylene. After thereaction was completed, the resultant reaction solution was cooled to120° to induce separation of a catalyst layer from the reaction solutionto the bottom. The catalyst layer was removed.

Subsequently, the remaining reaction solution was further cooled to 80°C. and the cooled reaction solution and 10 g of sulfuric acid (purity ofnot less than 98 wt. %) added thereto were stirred for 20 minutes. Thistreatment with sulfuric acid induced separation of a resinous substancefrom the reaction solution. The resinous substance was separated fromthe reaction solution. The remaining reaction solution was cooled to 50°C. The cooled reaction solution and 200 g of water added thereto werestirred for 30 minutes. The water layer educed during the treatment wasremoved.

Finally, the organic layer was distilled under a vacuum of 30 mmHg ofexpel ortho-xylene. Consequently, there were obtained yellow needlecrystals of N-phenyl maleimide.

By high performance liquid chromatography, the crystals were found tohave purity of 99.6% by weight, indicating the yield thereof to be 81.1mol % based on the N-phenyl maleinamic acid.

EXAMPLE 34

The procedure of Example 33 was repeated, except that the removal of thecatalyst layer separated from the reaction solution to the bottom wasomitted.

The N-phenyl maleimide consequently obtained was found to have purity of99.5% by weight, indicating the yield thereof to be 80.5 mol %.

EXAMPLE 35

In a flask provided with the same devices as in Example 32, 100 g ofortho-xylene and 100 g of maleic anhydride were heated by elevating theinternal temperature of the flask to 70° C. to dissolve maleic anhydridein the solvent.

Then, a solution of 124 g of o-chloroaniline in 870 g of ortho-xylenewas added thereto dropwise while under stirring over a period of 30minutes to synthesize a slurry solution of o-chlorophenyl maleinamicacid in ortho-xylene.

Then, the aforementioned slurry solution and 20 g ortho-phosphoric acid(85 wt. % aqueous solution), 0.07 g of zinc acetate, and 0.2 g ofp-methoxyphenol added thereto were heated and stirred at 140° C. forthree hours, with the water formed by the reaction removed from thereaction solution by azeotropic distillation with ortho-xylene. Afterthe reaction was completed, the resultant reaction solution was cooledto 100° C. to induce separation of a catalyst layer from the reactionsolution to the bottom. The catalyst layer was removed from the reactionsolution.

Subsequently, the remaining reaction solution was further cooledreaction solution and 15 g of sulfuric acid (purity of not less than 98%by weight) added thereto were stirred for 30 minutes. By this treatmentwith sulfuric acid, a resinous substance was educed from the reactionsolution. The resinous substance was removed from the reaction solution.The remaining reaction solution and 200 g of water added thereto werestirred at 30° C. for 15 minutes to induce separation of a water layerfrom the reaction solution.

The organic layer consequently obtained was distilled under a vacuum of30 mmHg to expel ortho-xylene and obtain o-chlorophenyl maleimide.

By high performance liquid chromatography, this product was found tohave purity of 99.7% by weight, indicating the yield thereof to be 80.1mol % based on o-chloroaniline as the raw material.

EXAMPLE 36

In a flask provided with the same devices as in Example 32, 100 g of asolvent containing not less than 98% of a petroleum fraction of aromatichydrocarbon having boiling points of 190° to 220° C. and 100 g of maleicanhydride were heated by elevating the internal temperature of the flaskto 70° C. to dissolve maleic anhydride in the solvent.

Then, a solution of 124 g of o-chloroaniline in 870 g of the samesolvent as mentioned above was added thereto dropwise while understirring over a period of 30 minutes to synthesize a slurry solution ofo-chlorophenyl maleinamic acid in ortho-xylene.

Then, the aforementioned slurry solution and 10 g of ortho-phosphoricacid (85 wt. % aqueous solution), 0.07 g of zinc acetate, and 0.5 g oftert-butyl catechol added thereto were heated and stirred at 185° C. for1 hour, with the water formed by the reaction removed by azeotropicdistillation with the solvent. After the reaction was completed, theresultant solution was cooled to 100° C. to induce separation of acatalyst layer from the reaction solution. The catalyst layer wasremoved from the reaction solution.

Subsequently, the remaining reaction solution was further cooled to 30°C. and the cooled reaction solution and 10 g of methane sulfonic acidadded thereto were stirred for 30 minutes. By the treatment with methanesulfonic acid resinous substance was educed from the reaction solution.The resinous substance was removed from the reaction. The reactionsolution and 200 g of water added thereto were stirred at 30° C. for 15minutes to effect separation of the water phase.

The organic layer consequently obtained was distilled under a vacuum of30 mmHg to produce o-chlorophenyl maleimide.

The o-chlorophenyl maleimide was found by high performance liquidchromatography to have purity of 99.5% by weight, indicating the yieldthereof to be 85.6 mol % based on o-chloroaniline as the raw material.

EXAMPLE 37

A glass flask having an inner volume of 10 liters was fitted with athermometer, a stirrer, and a water separator. A solution of 530 g ofmaleic anhydride in 500 g of xylene was placed in the glass flask. Then,into the glass flask whose interior temperature was adjusted in advanceto 130° C., a solution of 500 g of aniline in 4000 g of xylene was addedgradually and piecemeal over a period of 30 minutes to synthesize aslurry solution of N-phenyl maleinamic acid in xylene.

The slurry solution so obtained and 200 g of ortho-phosphoric acid, 0.35g of zinc acetate, and 1 g of copper dibutyl dithiocarbamate addedthereto were heated at 140° C. for three hours. Then, the resultantreaction solution was cooled to 30° C., washed with 1000 g of water, anddistilled under a vacuum to expel xylene. Consequently, there wasobtained 830 g of yellow N-phenyl maleimide. It was found to contain0.03% by weight of maleic acid, 0.20% by weight of maleinamic acid, and5 ppm of phosphoric acid.

The crude N-phenyl maleimide and 1 g of copper dibutyl dithiocarbamateadded thereto were heated and stirred by elevating the internaltemperature of the flask to 160° C. The resultant reaction solution wasdistilled under a vacuum of 3 mmHg for two hours to produce 773 g ofbright yellow N-phenyl maleimide crystals. The crystals were found tohave purity of 99.7% by weight, indicating the yield thereof bydistillation to be 98.0%.

CONTROL 5

The procedure of Example 37 was repeated, except that the washingtreatment with water was omitted. Consequently, there were obtained 623g of yellow crystals. These crystals were found to contain 1.0% byweight of maleic acid, 0.2% by weight of N-phenyl maleinamic acid, and0.05% by weight of phosphoric acid. The yield by distillation was 78.1%.

CONTROL 6

The procedure of Example 37 was repeated, except that the washingtreatment with water was omitted and the addition of the stabilizer inpreparation for the distillation was omitted. Consequenlty, there wereobtained yellow crystals. These crystals were found to have purity of98.3% by weight. The yield by distillation was 60.0%. In this case, aresinous residue occurred in a large amount in the distillation kettle.Thus, the content of the kettle defied stirring.

EXAMPLE 38

In a vertical reaction tank measuring 600 cm in inside diameter and 800cm in height and provided with a thermometer, a heating jacket, and acondenser incorporating a water separator, a mixed solution of 15 kg ofmaleic anhydride and 30 kg of xylene was placed, with the internaltemperature of the reaction tank adjusted to 70° C.

Then, a mixed solution containing 13.5 kg of aniline and 70 kg of xylenewas added thereto gradually and piecemeal over a period of 100 minutes.Consequently, there was obtained a slurry solution containing whitecrystals of N-phenyl maleinamic acid.

Subsequently, 3.2 kg of ortho-phosphoric acid, 9.6 g of zinc aceticacid, and 26.7 g of methoxy benzoquinone were added. By means of theheating jacket, the resultant reaction solution was heated to and keptat 136° C. for six hours to effect reaction. Thereafter, the reactionsolution was distilled under a vacuum of 30 mmHg (abs) to expel xylene.Consequently, there were obtained 24 kg of brown crystals of N-phenylmaleimide. The crystals were found to contain 0.1% by weight of N-phenylmaleinamic acid, 1.3% by weight of maleic acid, and 200 ppm ofphosphoric acid.

The crystals were comminuted and washed twice with 50 kg of water at 30°C. The resultant washed powder was found to contain 0.005% by weight ofN-phenyl maleinamic acid, 0.005% by weight of maleic acid, and not morethan 1 ppm of phosphoric acid.

The N-phenyl maleimide consequently obtained and 25 g of copper dibutyldithiocarbamate added thereto were heated and stirred by elevating theinternal temperature of the reaction tank to 165° C. The resultantreaction solution was distilled under a vacuum of 3 mmHg (abs) for fourhours. Consequently, there were obtained 21.5 kg of bright yellowN-phenyl maleimide crystals. The crystals were found to have purity of99.5% by weight, indicating the yield thereof by distillation to be98.2%.

EXAMPLE 39

In a flask provided with a thermometer, a condenser incorporating amoisture separator, a dropping funnel, and a stirrer, 100 g of p-cymeneand 100 g of maleic anhydride added thereto were heated by elevating theinternal temperature of the flask to 100° C. to dissolve the maleicanhydride.

Subsequently, a solution of 100 g of cyclohexylamine in 600 g ofp-cymene was added thereto dropwise while under stirring to synthesize aslurry solution of N-cyclohexyl maleinamic acid in p-cymene.

Then, the slurry solution and 80 g of orthophosphoric acid, and 0.1 g ofcopper dibutyl dithiocarbamate added thereto were heated and stirred forseven hours, with the water formed by the reaction removed by azeotropicdistillation with the solvent. After the reaction was completed, thereaction solution was held at 180° C. to induce separation of an acidcatalyst from the reaction solution to the bottom. This catalyst layerwas removed from the reaction solution.

Subsequently, the reaction solution was cooled to 60° C. and the cooledreaction solution and 100 g of water added thereto were stirred for 30minutes to induce separation of a water layer. This washing treatmentwas repeated once more. The organic layer consequently obtained wasdistilled under a vacuum of 10 mmHg (abs) to expel the solvent.

Then, the reaction solution held in the flask and 0.3 g of copperdibutyl dithiocarbamate newly added thereto were held under a vacuum of5 mmHg (abs) for 30 minutes with the internal temperature kept withinthe range of 130° to 150° C. to effect distillation of N-cyclohexylmaleimide. As the result, there were obtained 162 g of bright whiteN-cyclohexyl maleimide crystals. The crystals were found to have purityof 99.8% by weight, indicating the yield thereof to be 89.5 mol % basedon cyclohexylamine as the raw material.

What is claimed is:
 1. In a method for the production of N-phenyl andN-cyclohexyl maleimides produced by subjecting the correspondingmaleinamic acid to ring-closure imidation in an organic solvent capableof forming an azeotrope with water in the presence of an acid catalystat a temperature in the range of 120° to 250° C. while removing theformed water in the form of an azeotrope with said organic solventfollowed by purification which comprises washing the reaction mixturewith water, removing the solvent, and distilling the washed residuewherein the improvement is the use during the distillation step of atleast one stabilizer selected from the group consisting of quinones,phenols, thiodipropionoic acid esters, di-thiocarbamates, salicylates,alkyldiphenyl-amides, phenothiazines, mercaptoimidazoles and triphenylphosphates.
 2. A method according to claim 1 wherein an amount of saidstabilizer is not less than 10 ppm (by weight) to said washed residue.3. A method according to claim 1 wherein the amount of said stabilizeris 100 to 2,000 ppm (by weight) to said washed residue.
 4. A methodaccording to claim 1 wherein said washing with water is carried outuntil an acid content in said washed residue decreases to not more than0.5% by weight.
 5. A method according to claim 1, wherein saidstabilizer is dithiocarbamates.
 6. A method according to claim 5,wherein said stabilizer is copper dithiocarbamate.
 7. A method accordingto claim 1, wherein said aromatic substituted maleimides are N-phenylmaleimide.
 8. A method according to claim 1, wherein the amount of saidstabilizer is 200 to 1,000 ppm (by weight) to said washed residue.
 9. Amethod according to claim 1, wherein said washing with water is carriedout until an acid content in said washed residue decreases to not morethan 0.1% by weight.